Sleep apnea, an airway disorder, has been known for some time as a medical syndrome in two generally recognized forms. The first is central sleep apnea, which is associated with the failure of the body to automatically generate the neuromuscular stimulation necessary to initiate and control a respiratory cycle at the proper time. Work associated with employing electrical stimulation to treat this condition is discussed in Glenn, "Diaphragm Pacing: Present Status", Pace, V.I, pp 357-370 (July-September 1978).
The second sleep apnea syndrome is known as obstructive sleep apnea. Ordinarily, the contraction of the dilator muscles of the upper airways (nose and pharynx) allows their patency at the time of inspiration. In obstructive sleep apnea, the obstruction of the airways results in a disequilibrium between the forces which tend to collapse airways (negative inspiratory transpharyngeal pressure gradient) and those which contribute to their opening (muscle contraction). The mechanisms which underlie the triggering of obstructive apnea include a reduction in the size of the superior airways, an increase in their compliance, and a reduction in the activity of the muscle dilator. The muscle dilators are intimately linked to the respiratory muscles and these muscles respond in a similar manner to a stimulation or a depression of the respiratory center. The ventilatory fluctuations observed during sleep (alternately hyper and hypo ventilation of periodic respiration) thus favors an instability of the superior airways and the occurrence of oropharyngeal obstruction. In sleep apnea the respiratory activation of the genioglossus muscle has been particularly noted to be ineffective during sleep. The cardiovascular consequences of apnea include disorders of cardiac rhythm (bradycardia, auriculoventricular block, ventricular extrasystoles) and hemodynamic (pulmonary and systemic hypertension). This results in a stimulatory metabolic and mechanical effect on the autonomic nervous system. The syndrome is therefore associated with an increased morbidity (the consequence of diurnal hypersomnolence and cardiovascular complications).
A method for treatment of sleep-apnea syndrome is to generate electrical signals to stimulate those nerves which activate the patient's upper airway muscles in order to maintain upper airway patency. For example, in U.S. Pat. No. 4,830,008 to Meer, inspiratory effort is monitored and electrical signals are directed to upper airway muscles in response to the monitored inspiratory effort. Or, for example, in U.S. Pat. No. 5,123,425 to Shannon, Jr. et al., a collar contains a sensor to monitor respiratory functioning to detect an apnea episode and an electronics module which generates electrical bursts to electrodes located on the collar. The electrical bursts are transferred transcutaneously from the electrodes to the nerves innervating the upper airway muscles. Or, for example, in U.S. Pat. No. 5,174,287 issued to Kallok, sensors monitor the electrical activity associated with contractions of the diaphragm and also the pressure within the thorax and the upper airway. Whenever electrical activity of the diaphragm suggests that an inspiration cycle is in progress and the pressure sensors show an abnormal pressure differential across the airway, the presence of sleep apnea is assumed and electrical stimulation is applied to the musculature of the upper airway. Or, for example, in U.S. Pat. No. 5,178,156 issued to Wataru et al., respiration sensing includes sensors for sensing breathing through left and right nostrils and through the mouth which identifies an apnea event and thereby triggers electrical stimulation of genioglossus muscle. Or, for example, in U.S. Pat. No. 5,190,053 issued to Meer, an intra-oral, sublingual electrode is used for the electrical stimulation of the genioglossus muscle to maintain the patency of an upper airway. Or, for example, in U.S. Pat. No. 5,211,173 issued to Kallok et al., sensors are used to determine the effectiveness of the stimulation of the upper airway and the amplitude and pulse width of the stimulation are modified in response to the measurements from the sensors. Or, for example, in U.S. Pat. No. 5,215,082 issued to Kallok et al., upon sensing of the onset of an apnea event, a stimulation generator provides a signal for stimulating the muscles of the upper airway at a varying intensity such that the intensity is gradually increased during the course of the stimulation. Or, for example, in U.S. Pat. No. 5,483,969 issued to Testerman et al., stimulation of an upper airway muscle is synchronized with the inspiratory phase of a patient's respiratory cycle using a digitized respiratory effort waveform. A fully implantable stimulation system is described in Testerman et al. with a sensor implanted in a position which has pressure continuity with the intrapleural space such as the suprasternal notch, the space between the trachea and esophagus or an intercostal placement.
However, even with these modes of respiratory disorder treatment, there remain many practical difficulties for implementing them and other therapy treatments in medically useful systems. In particular, if therapy systems, such as, for example, systems for treatment of respiratory disorders, depend upon use of a signal characteristic of a periodic physiological parameter, such a signal should be continuously provided in an accurate, reliable and normalized manner. For example, this is particularly important if the therapy by the system is based on comparisons of the signal relative to thresholds set for the signal. Although various gain control devices and methods are available, there is a need in the art for additional and/or improved gain control methods and devices for providing such normalized signals.